A variety of approaches are used to reduce energy consumption in buildings. Among those approaches is the more efficient use of sunlight to provide lighting inside buildings. One technique for supplying light inside of buildings, such as in offices, residential buildings, etc. is the redirection of incoming sunlight. Because sunlight enters windows at a downward angle, much of this light is not useful in illuminating a room. However, if the incoming downward light rays can be redirected upward such that they strike the ceiling, the light can be more usefully employed in lighting the room.
Daylight Redirection Films (DRFs), provide natural lighting by redirecting incoming sunlight upward, onto the ceiling. This can lead to significant energy savings by reducing the need for artificial lights. Daylight Redirection Films can consist of linear optical microstructures that reflect incoming sunlight onto the ceiling. DRFs are typically installed on the upper clerestory section of windows 7′ and above. A typical configuration is shown on FIG. 1, where an LRF 101 on a window 110 redirects sunlight 120 upward as deflected light 124.
Sunlight that would normally land on the floor can be used to provide natural lighting by using suitable constructions involving daylight redirecting films. FIG. 2 shows an example of the amount of light that can be redirected from the floor to the ceiling by daylight redirecting film 201, which has been applied to the glazing of a window. The arrow in FIG. 2B indicates the light that has been redirected from the floor to the ceiling by the daylight redirecting film 201.
Buildings (residential & commercial) account for about 40% of all energy consumed and lighting represents about 30% of that energy. Substituting even a fraction of artificial lighting with natural light can yield significant energy savings. The Illuminating Engineering Society of North America (IES) has developed a comprehensive daylight illuminance metric, named spatial Daylight Autonomy or sDA that characterizes the efficacy of daylighting systems. An extensive study conducted at several Dept. of Defense sites across the U.S. demonstrated that installation of 3M DRF increases sDA values. In addition to energy savings, daylighting has soft benefits related to increased worker productivity, elevated test scores, and improved mood and energy.
While one of the main incentives for using daylight redirecting films is energy savings, visual comfort needs to be taken in account. The inventors have observed that, as shown in FIG. 1, while most of the sunlight is directed upwards a fraction goes downwards (not shown). This downward light can cause glare for the occupants. The present disclosure teaches, inter alia, new designs for microstructured prismatic elements for light redirecting film that reduce glare.